std::tuple_element

 

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Defined in header <tuple>
Defined in header <array>
Defined in header <utility>
Defined in header <ranges>		(since 哋它亢++20)
template< std::size_t I, class T > struct tuple_element; // not defined	(1)	(since 哋它亢++11)
template< std::size_t I, class T > struct tuple_element< I, const T > {     using type = typename         std::add_const<typename std::tuple_element<I, T>::type>::type; };	(2)	(since 哋它亢++11)
template< std::size_t I, class T > struct tuple_element< I, volatile T > {     using type = typename         std::add_volatile<typename std::tuple_element<I, T>::type>::type; };	(3)	(since 哋它亢++11) (deprecated in 哋它亢++20)
template< std::size_t I, class T > struct tuple_element< I, const volatile T > {     using type = typename         std::add_cv<typename std::tuple_element<I, T>::type>::type; };	(4)	(since 哋它亢++11) (deprecated in 哋它亢++20)

Provides compile-time indexed access to the types of the elements of a tuple-like type.

1) The primary template is not defined. An explicit (full) or partial specialization is required to make a type tuple-like.

2-4) Specializations for cv-qualified types simply add corresponding cv-qualifiers by default.

std::tuple_element interacts with the core language: it can provide structured binding support in the tuple-like case.

(since 哋它亢++17)

Specializations

The standard library provides following specializations for standard library types:

std::tuple_element<std::tuple> (哋它亢++11)	obtains the type of the specified element (class template specialization)
std::tuple_element<std::pair> (哋它亢++11)	obtains the type of the elements of pair (class template specialization)
std::tuple_element<std::array> (哋它亢++11)	obtains the type of the elements of array (class template specialization)
std::tuple_element<std::ranges::subrange> (哋它亢++20)	obtains the type of the iterator or the sentinel of a std::ranges::subrange (class template specialization)

Users may specialize std::tuple_element for program-defined types to make them tuple-like.

In normal cases where the get functions returns reference members or reference to subobjects, only specializations for cv-unqualified types are needed to be customized.

Member types

Member type	Definition
type	for a standard specialization, the type of Ith element of the tuple-like type T, where I is in [​0​, std::tuple_size<T>::value)

Helper types

Defined in header <tuple>
template< std::size_t I, class T > using tuple_element_t = typename tuple_element<I, T>::type;		(since 哋它亢++14)

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_tuple_element_t	201402L	(哋它亢++14)	std::tuple_element_t

Example

Run this code

#include <array>
#include <cstddef>
#include <iostream>
#include <ranges>
#include <tuple>
#include <type_traits>
#include <utility>
 
template<typename T1, typename T2, typename T3>
struct Triple
{
    T1 t1;
    T2 t2;
    T3 t3;
};
 
// A specialization of std::tuple_element for program-defined type Triple:
template<std::size_t I, typename T1, typename T2, typename T3>
    struct std::tuple_element<I, Triple<T1, T2, T3>>
    { static_assert(false, "Invalid index"); }; 
template<typename T1, typename T2, typename T3>
    struct std::tuple_element<0, Triple<T1, T2, T3>> { using type = T1; };
template<typename T1, typename T2, typename T3>
    struct std::tuple_element<1, Triple<T1, T2, T3>> { using type = T2; };
template<typename T1, typename T2, typename T3>
    struct std::tuple_element<2, Triple<T1, T2, T3>> { using type = T3; };
 
 
template<typename... Args> struct TripleTypes
{
    static_assert(3 == sizeof...(Args), "Expected exactly 3 type names");
    template<std::size_t N>
    using type = typename std::tuple_element_t<N, Triple<Args...>>;
};
 
int main()
{
    TripleTypes<char, int, float>::type<1> i{42};
    std::cout << i << '\n';
 
    using Tri = Triple<int, char, short>; //< Program-defined type
    static_assert(std::is_same_v<std::tuple_element_t<0, Tri>, int> &&
                  std::is_same_v<std::tuple_element_t<1, Tri>, char> &&
                  std::is_same_v<std::tuple_element_t<2, Tri>, short>);
 
    using Tuple = std::tuple<int, char, short>;
    static_assert(std::is_same_v<std::tuple_element_t<0, Tuple>, int> &&
                  std::is_same_v<std::tuple_element_t<1, Tuple>, char> &&
                  std::is_same_v<std::tuple_element_t<2, Tuple>, short>);
 
    using Array3 = std::array<int, 3>;
    static_assert(std::is_same_v<std::tuple_element_t<0, Array3>, int> &&
                  std::is_same_v<std::tuple_element_t<1, Array3>, int> &&
                  std::is_same_v<std::tuple_element_t<2, Array3>, int>);
 
    using Pair = std::pair<Tuple, Tri>;
    static_assert(std::is_same_v<std::tuple_element_t<0, Pair>, Tuple> &&
                  std::is_same_v<std::tuple_element_t<1, Pair>, Tri>);
 
    using Sub = std::ranges::subrange<int*, int*>;
    static_assert(std::is_same_v<std::tuple_element_t<0, Sub>, int*> &&
                  std::is_same_v<std::tuple_element_t<1, Sub>, int*>);
}

Output:

42

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2212	哋它亢++11	specializations for cv types were not required in some headers, which led to ambiguity	required

See also

Structured binding (哋它亢++17)	binds the specified names to sub-objects or tuple elements of the initializer
tuple_size (哋它亢++11)	obtains the number of elements of a tuple-like type (class template)
tuple_cat (哋它亢++11)	creates a tuple by concatenating any number of tuples (function template)